Organic electroluminescent devices are made from materials that emit light when a suitable voltage is applied across electrodes deposited on either side of the organic material. One class of such materials is semiconductive conjugated polymers which have been described in our earlier U.S. Pat. No. 5,247,190, the contents of which are herein incorporated by reference.
One particular advantage such devices have over traditional inorganic light emitting diodes is the ease with which they can be patterned to produce areas of light emission. This is extremely important for the fabrication of dot-matrix displays. The speed of switching of organic light emitting diodes is limited by the transport time of carriers (electrons and holes) moving from their respective electrodes into the material where they combine to form excitons which can radiatively decay to produce light. Because the organic layers can be made very thin (typically &lt;1 micron e.g. 0.1 micron) this transport time can be of order 0.1-1 microsecond even though the mobility of carriers in the organic materials is considerably less (by several orders of magnitude) than the mobility in traditional semiconductors.
One of the primary consequences of this relatively low mobility is a limit to the peak current that can be injected into the device. This is because of space charge effects. Such effects lead to an accumulation of charge within the devices which reduce the further injection of carriers. This has been studied recently in organic LEDs, for example in P. Blom et al, Appl Phys. Lett. p 3308, Vol 68, 1996. The present inventors have reasoned that higher mobilities would lead to higher peak currents and therefore higher peak brightnesses and faster switching times. This is important for many applications including time multiplexed displays. High injection currents are also likely to be required for operation of electrically pumped lasers. If the increase in mobility also leads to a balancing of charge injection this will have the additional benefit of increasing the emission efficiency of these devices.
Attempts have been made to operate electroluminescent devices in a pulsed mode. For example, reference is made to D, Braun et al, Appl. Phys. Lett., p3092, vol. 61, 1992. In that paper, 1 .mu.s voltage pulses of 40V were applied to an EL device. It is stated that at a duty cycle of 0.5% (1:200), the EL intensity remained proportional to the current up to 10 A/sqcm. We believe this is still much too low to have any significant effect on mobility.
In F. Hide et al in Science, p1833, vol.273, 1996, a possibility of obtaining current densities of 25 A/sqcm when operated with 3 .mu.s pulses at a low duty cycle of 1:30 is discussed. We believe again that this is too low to have any significant effect on light output or emission efficiency.
In a paper by L. Rothberg et al, Synthetic Metals, p41, vol.80, 1996, it is suggested that a theoretical maximum value for peak current injection would be 200 A/sqcm. However this is based on expectations for carrier mobility in PPV and removal of space charge limits which are wholly unrealistic in the context described by Rothberg.
It is an object of the present invention to provide a pulsed mode of operation of an EL device which significantly exceeds any existing realistic expectations of current density.